Known in the art is an internal combustion engine arranging in an engine exhaust passage an NOx storage catalyst storing NOx contained in exhaust gas when the air-fuel ratio of the inflowing exhaust gas is lean and releasing the stored NOx when the air-fuel ratio of the inflowing exhaust gas becomes a stoichiometric air-fuel ratio or rich. In this internal combustion engine, NOx formed when burning fuel under a lean air-fuel ratio is stored in the NOx storage catalyst. On the other hand, as the NOx storage catalyst approaches saturation of the NOx storage ability, the air-fuel ratio of the exhaust gas is temporarily made rich, whereby NOx is released from the NOx storage catalyst and reduced.
However, fuel and lubrication oil contain sulfur. Therefore, the exhaust gas also contains SOx. This SOx is stored together with the NOx in the NOx storage catalyst. This SOx is not released from the NOx storage catalyst by just making the exhaust gas a rich air-fuel ratio. Therefore, the amount of SOx stored in the NOx storage catalyst gradually increases. As a result, the storable NOx amount ends up gradually decreasing.
Therefore, known in the art is an internal combustion engine preventing SOx from being fed into the NOx storage catalyst by providing an SOx absorbent in the engine exhaust passage upstream of the NOx storage catalyst (see Japanese Patent Publication (A) No. 2000-179327). In this internal combustion engine, the SOx contained in the exhaust gas is absorbed by the SOx absorbent, therefore the SOx is prevented from flowing into the NOx storage catalyst. As a result, it is possible to prevent the storage of SOx from causing a drop in the storage ability of the NOx.
However, when using such an SOx absorbent, if the SOx absorbent ends up being saturated in SOx absorption ability, the SOx ends up flowing into the NOx storage catalyst. However, in this SOx absorbent, if raising the SOx absorbent in temperature and making the exhaust gas flowing into the SOx absorbent a rich air-fuel ratio, it is possible to make the SOx absorbent release the absorbed SOx and therefore possible to regenerate the SOx absorbent. However, if making the SOx absorbent release the SOx in this way, the released SOx ends up being stored in the NOx storage catalyst. Therefore, in this internal combustion engine, a bypass passage bypassing the NOx storage catalyst is provided and, when making the SOx absorbent release SOx, the released SOx is exhausted through the bypass passage into the atmosphere.
However, even if exhausting the SOx released from the SOx absorbent through the bypass passage into the atmosphere at the time of regeneration of the SOx absorbent in this way, in actuality part of the SOx leaks and ends up flowing into the NOx storage catalyst. As a result, there is a problem that the amount of SOx stored in the NOx storage catalyst gradually increases.